1. Field Of The Invention
The present invention relates generally to control systems for AC induction motors and, more particularly, to an apparatus and method for providing signals, indicative of the magnitude and angular position of the motor flux of an AC induction motor, to control the motor drive current.
2. Prior Art
The AC induction motor is increasingly being used as a source of controllable rotation and torque. Such an AC induction motor can be driven by an output or drive current signal of variable magnitude and frequency to produce a desired rotation and torque.
As is well known, the drive current is supplied to the stator windings of the AC induction motor. The drive current in the stator windings creates flux in the motor, which causes torque to be produced when the drive current in the stator windings is at an angle to the flux field of the rotor of the motor.
The level of flux is an important parameter to measure and use to control the generation of the drive current to ensure that the motor produces the desired rotation and torque. Typically, two flux measurements are used to achieve desired control of the generation of the drive current: the first axis flux signal proportional to the first axis component of the flux produced by the drive current in the stator windings; the second axis flux signal proportional to the second axis component of the flux produced by the drive current in the stator windings. Often, the first axis and the second axis are substantially orthogonal with respect to each other. The first and second axes flux signals allow the magnitude and angular position of the motor flux in two dimensions to be known at each instant.
Conventional apparatus and methods can provide signals indicative of the magnitude and angular position of the motor flux at any instant when the motor is in the rotating state. However, it is often desirable to operate the motor in the substantially non-rotating state, either at substantial torque or at substantially zero torque. Examples of where such operation is desirable are manifold, including when the motor is required to stop and maintain the load torque at a fixed position.
When the motor is in the substantially non-rotating state, however, it becomes very important to maintain the values of the flux signals indicating the magnitude and angular position of the non-rotating, or DC, motor flux so that when the motor is taken out of the nonrotating state, undesirable motor response does not occur.
As is well known, the frequency of the drive current is typically substantially zero when the motor is in the substantially non-rotating state at zero torque. For this reason, the flux present in the motor is termed DC flux when the frequency of the drive current is substantially zero.
A Hall device is a conventional sensor which provides an output signal proportional to the level of the DC flux. As is well known, however, the Hall device is extremely fragile, is highly susceptible to damage caused by mechanical shock and vibrations, and is expensive to manufacture.
Several other conventional approaches allow the measurement of the magnitude and angular position of the motor flux when the motor is not in the substantially non-rotating state. Such approaches include measuring the rate of change of motor flux in accordance with the terminal voltages of the motor and sensing the rate of change of motor flux using flux coils. With both of these approaches, the level of the signal proportional to the rate of change of motor flux can be calculated using the following formula: EQU e=Nd.psi./dt,
where:
e=the voltage level of the rate of change of motor flux signal; PA1 N=constant; and PA1 d.psi./dt=rate of change of the stator flux of the motor.
As is apparent, the value for d.psi./dt goes to zero as the frequency of the drive current goes to zero: the magnitude of the e signal goes to zero when the frequency goes to zero. Thus, the two conventional approaches which measure rate of change of motor flux do not provide a signal indicative of the DC flux when the frequency of the drive current is substantially zero, even when the magnitude of the DC flux is substantial.
An additional problem with conventional apparatus and methods for providing control signals indicative of the magnitude and angular position of the motor flux when the frequency of the drive current is substantially zero is that the integrators used to integrate the e signals to produce the flux signals cannot maintain, over a substantial time period, the value of the motor flux immediately prior to the time when the frequency went to zero. Thus, the flux signals provided when DC flux is present tend to vary due to undesirable drift present in conventional integrators, causing the flux signals to be incorrect when the motor is taken out of the non-rotating state. This incorrect value for the DC flux causes undesirable responses by the motor when the frequency is again made greater than zero.